Discontinuous fibrillated fibers are preferentially employed as a raw material for obtaining threads or sheet-form material such as non-woven cloth or the like: such fibers are represented by pulp. Recently, in fields requiring a high filtration ability with low pressure loss, such as air filters and the like, the effective use of extremely thin fibers having a large surface area has been required. The use of fibrillated fibers has been proposed to increase the surface area and raise the filtration efficiency.
A large number of manufacturing methods for discontinuous fibrillated fibers used as materials in non-woven cloth, paper, and the like, have been known.
For example, in Japanese Patent Application, Second Publication No. Sho 35-11851, a method is discussed in which, when a polymer solution is discharged into a coagulation bath, and the polymer is precipitated and coagulated, the polymer granules, which are in a swollen state, or the fibrous materials, which are in a swollen state, are subjected to deformation or beating by means of appropriate shearing action, and thereby, a pulp material containing fibrillated fibers is obtained. The use of high speed agitation using an agitator having an angle at the rotational surface of a paddle or a blade, or alternatively, the discharge of the polymer solution and air simultaneously into the coagulation bath from a two-fluid nozzle, are disclosed as methods for applying shear.
However, the pulp material obtained by means of such a method is in a fibrillar shape having a plurality of tentacle-shaped projections, the smallest dimension of which does not exceed 10 microns, or is in a thin film shape or a ribbon shape, so that the shape thereof is insufficiently controlled as a fibrillated fiber structure.
The flash spinning method disclosed in Japanese Patent Application, First Publication No. Sho 40-28125 and in Japanese Patent Application, First Publication No. Sho 41-6215, is known as a method for producing continuous fibers (plexifilaments) of a large number of fibrillated fibers.
In this spinning method, a crystalline polymer solution which is at a temperature higher than the standard boiling point of the solution and in the spontaneous vapor pressure region or at a pressure higher than this is extruded into a low pressure region from an appropriately shaped orifice, and thereby, the solvent volatilizes violently, and the majority of the extruded polymers are torn, and thereby, continuous fibrillated fibers are formed. This method requires the instantaneous volatilization of the solvent, so that it is necessary to employ a solvent having a comparatively low boiling point, for example, benzene, toluene, cyclohexane, methylene chloride, or the like, and furthermore, it is necessary to select a polymer which forms a uniform solution in the solvent employed under high temperature and high pressure conditions, and which, moreover, is not soluble in this solvent when extruded into a low pressure region, so that the composition of the fibrillated fibers obtained is limited.
Furthermore, this method involves the use of low boiling point solvents, and the maintenance of high pressure and high temperature states, so that it is not industrially advantageous. Furthermore, the fibers obtained are plexifilaments, and it is difficult to form discontinuous fibrillated fibers using such a method.
Improvements to the flash spinning technology which serve as methods for producing discontinuous fibers are disclosed in Japanese Patent Application, Second Publication No. Sho 48-1416, Japanese Patent Application, Second Publication No. Sho 54-39500, and Japanese Patent Application, First Publication No. Hei 6-207309.
A method for obtaining fibrillated fibers by extruding an aqueous dispersion solution, obtained by dispersing a molten polymer in a large amount of water, together with additional water into a low pressure region is disclosed in Japanese Patent Application, Second Publication No. Sho 48-1416.
However, in this method, it is necessary to employ an extruder having a special structure because the polymer is dispersed in a large amount of water, and this can not be accomplished easily.
A method for obtaining discontinuous fibrillated fibers, in which continuous fibrillated fibers are obtained by the sudden lowering of pressure on a mixture of two-liquids, a molten polymer, and a solvent, these are torn by means of a water vapor flow, and the fibers are thus torn, is disclosed in Japanese Patent Application, Second Publication No. Sho 54-39500. A method is disclosed in Japanese Patent Application, First Publication No. Hei 6-207309 in which an inert fluid is brought into contact with flash-spun fibers, and by means of the appropriate adjustment of the volumetric flow rate of the inert fluid and the solvent vapor, discontinuity is achieved.
However, these methods also involve high-pressure operations.
A method which serves to reduce these high pressures is disclosed in Japanese Patent Application, First Publication No. Sho 51-19490; in this method, a solution of a thermoplastic polymer and a solvent is formed at a pressure below the critical solution pressure and a temperature below the low temperature critical solution temperature, and an emulsion employing this solution as a dispersoid and water as a dispersant is sprayed into a low pressure region together with a pressurized gas using a two-fluid nozzle.
However, although the pressure is lower in this method, it is still necessary to maintain the emulsion at a pressure within a range of 10-20 atmospheres.
A manufacturing method for pulp materials which does not require the use of high pressures has been disclosed in Japanese Patent Application, First Publication No. Sho 61-12912; in this method, an aromatic polyamide is dissolved in sulfolane, and this solution is dispersed using a high temperature gas under conditions generating high shearing forces. In this method, the use of a two-fluid nozzle, and the use of water as the high temperature gas, are proposed.
However, the viscosity of the polymer solution which is employed in this method is within a range of from 10 cP to 10.sup.5 cP, and this is low in comparison with the viscosity of polymer solutions employed in the wet spinning of common fibers, so that this method is difficult to use for widely used polymers. Furthermore, the substances obtained are in pulp form, and are not appropriate for use in non-woven cloths which are employed in filter applications and the like.
Furthermore, a method is disclosed in Japanese Patent Application, First Publication No. Hei 2-234909 for manufacturing sub-denier fibers from lyotropic liquid crystal polymers. In this method, an optically anisotropic polymer solution is extruded into a chamber, and in this chamber, a pressurized gas flows around the polymer and in contact therewith, and this moves in the direction of flow, and the polymer and the gas both pass through a gap into a low pressure region, and while thinning this flow, passage is conducted at a sufficient speed to split into fibers, and in this region, the split flow is brought into contact with a coagulating fluid.
However, in this method, it is necessary to pass a high viscosity polymer solution coming out of an extrusion port through a further gap, and blockage of the gap by the polymer solution is likely to occur, so that this method is not industrially advantageous.
A melt blown spinning method used in industry for polyester fibers and the like is a method for producing fibers on the submicron order. In this method, a polymer in a molten state which is extruded by an extruder is caused to lengthen, thin, and solidify in a high-speed gas flow, and submicron order fibers are obtained.
However, in this method, a thermally meltable polymer is a prerequisite, so that the method is not appropriate for use with polymers having a high melting temperature or polymers which are thermally deformable.
There is also a method in which islands-in-a-sea spinning of a polymer having two components having differing solution characteristics is conducted, and the island components are eluted, to produce ultrathin fibers.
However, in this method, after the fibers have been produced, it is necessary to elute the island components, and this is not economical. Furthermore, it is presently difficult to spin minute islands-in-a-sea type fibers using solution spinning, which is a spinning method for macromolecular substances which do not thermally melt.
In Japanese Patent Application, Second Publication No. Sho 52-18291, a method is disclosed in which a mixture comprising two or more thermoplastic resins which are hydrophobic and mutually insoluble, or this mixture with inorganic or organic material added thereto, is heated and melted, extruded through a slit nozzle, and after being drawn in one direction and formed into a band, the molecules whereof are oriented, chips obtained by cutting this band into lengths within a range of 3-50 mm are fibrillated by means of physical pressure, and by means of adding a water-soluble polymer, beating fibrillation is facilitated.
However, this method is applicable to thermoplastic resins; this method can not be applied to polymers such as cellulose, cellulose acetate, acrylonitrile polymers, and the like, which have a comparatively high melting point, are subject to thermal deformation, and are difficult to place in a molten state.
Solution spinning is used a manufacturing method for fibrillated fibers of polymers difficult to place in a molten state. In Japanese Patent Application, First Publication No. Hei 3-130411, which discloses a method for obtaining submicron order fibers of a polymer using this solution spinning, an ultrathin fiber having a diameter of 2 micrometer or less and an aspect ratio of 1,000 or more which comprises a polymer consisting of 85% or more acrylonitrile is disclosed. The method disclosed is one in which a mixed solution of polymers having different solubilities is prepared, and this solution is made into fibers by a commonly known spinning method, and after this, one polymer is eluted to produce an ultrathin fiber.
However, as in the case of the islands-in-a-sea type fiber described above, a polymer must be removed by elution, so that this is not economical, and in consideration of present-day environmental problems, it is necessary to solve the problem of the recovery or disposal of the eluted polymer solution, so that this is not an industrially advantageous method.
A manufacturing method for acrylonitrile type pulp is disclosed in Japanese Patent Application, First Publication No. Hei 3-104915, in which a solution containing 3-10 weight percent of a polymer having an average molecular weight of 300,000 or above, chiefly consisting of acrylonitrile, is wet spun, and formed into a fiber having a large number of pores, and subsequently an acrylonitrile pulp having fibrils with a diameter of 0.5 micrometers or less is obtained by beating.
However, in this method, even after beating, only a portion becomes fibers having a diameter of 0.5 micrometers or less, and the basic fibers remain, so that such pulp is insufficient for uses such as filters and the like which require a high surface area. Furthermore, when used for artificial leather and the like, the basic fibers have a deleterious effect on the feel, and this is not desirable.
A method for obtaining fibers having a submicron order diameter comprising a cellulose system polymer is disclosed in "Seni to Kougyou," Volume 48, Number 10 (1992), in which cellulose fibers are beaten in a high-pressure homogenizer. This method takes advantage of the highly crystalline characteristics of cellulose, and beating of the cellulose fibers, the fibrillation of which has proceeded, is continued to a microfibril order.
However, this method requires the use of a special device for the beating, so that it is not broadly applicable. Furthermore, the method may be applied to cellulose; however, it is difficult to apply the method to cellulose acetate or acrylonitrile system polymers, which are useful macromolecules not subject to thermal melting.